Aluminum hydroxide is produced on an industrial scale by well-established methods such as the Bayer process. The precipitation process operators optimize their methods so as to produce the greatest possible yield from the aluminate process liquors while trying to achieve a given crystal size distribution of aluminum hydroxide product. It is desirable in most instances to obtain the product of relatively large crystal size since this is beneficial in subsequent processing steps required to produce aluminum metal. Production is often limited by processing conditions under which the crystallization and precipitation is conducted. These processing conditions vary from one plant to the next and include, but are not limited to, temperature profiles, seed charge, seed crystal surface area, purge of carbon dioxide or flue gases, liquor loading, liquor purity, and the like.
Extensive efforts have been invested into finding chemical additives and methods limiting the factors negatively affecting particle size and yield in order to achieve the optimal economic recovery of aluminum hydroxide product. One of such factors is the presence of oxalate anion in the precipitation liquor. Sodium oxalate crystallizes and co-precipitates from solution over essentially the same temperature profiles, as does the desirable aluminum hydroxide product. If left undealt with, oxalate precipitation results in a decrease of the average particle size and yield of aluminum hydroxide product through a number of mechanisms. Oxalate crystals act as seed sites resulting in generation of undersized aluminum hydroxide crystals during the precipitation stage. Oxalate crystals adhere to the surfaces of growing aluminum hydroxide and incorporate within the precipitated product agglomerates. Thus created agglomerates disintegrate during the washing and calcination stages that follow. Also, under certain conditions, these agglomerates grow to significant sizes (sometimes greater than 0.5 inch) and accumulate at the bottom of precipitation vessels hindering mixing. The removal of these agglomerates results in shutdowns for cleaning as well as the loss of aluminum values.
Therefore, effective oxalate removal from the process is crucial for economical recovery of a high quality aluminum hydroxide product.
Washing with water the fine aluminum hydroxide returning to the process as seed is a common method of oxalate removal. Untreated precipitation liquors yield sodium oxalate crystals with needle like morphology that incorporate into the aluminum hydroxide product as mentioned above. Organic crystal growth modifiers are known to force oxalate crystallize as spherical agglomerates of such needles also known as “oxalate balls.” For the seed washing method, it is desirable that these balls do not overgrow the size that can be effectively dissolved in the duration of the washing stage.
Another common method of oxalate removal is the side-stream destruction. This method requires that oxalate does not crystallize during the precipitation stage, but rather is carried with the spent liquor until removal. Commonly in this method, oxalate is removed by precipitation in a side stream circuit, and therefore, it is also critical that a crystal growth modifier does not act as an oxalate precipitation poison.
Despite the continuous and ongoing development worldwide, the industry demands for economical resolution of the above-described process needs remain unfulfilled. A method of such resolution suitable for obtaining aluminum hydroxide crystals with increased particle size and yield, while facilitating oxalate removal is provided by the present invention.